The subject of the present invention is a process for preparing particles coming from the hydrolysis of a salt of a metal cation.
It is already known to prepare calcium phosphate crystals in the presence of copolymers having two hydrophilic blocks of different nature by alkyl chain anchoring (Antonietti, Mann, Chem. Eur. J., 1998). It is also known to control the growth of calcium carbonate crystals (J. M. Marentette, Adv. Materials, 1997; and M. Antonietti, Langmuir, 1998; as well as H. Colfen and M. Antoniette, Langmuir, 1998, 14, 582-589). It is also known to control the growth of noble metals in the presence of block copolymers having at least one hydrophilic block and at least one hydrophobic block (S. Foster and M. Antonietti, Adv. Materials, 1997).
The present invention provides a process for preparing mineral particles in the form of colloids in aqueous dispersion of controlled size from a precursor, comprising:
a) the step of putting the said precursor, which is a mineral salt of a metal cation, into aqueous solution;
b) the complexation of the product obtained in a) with at least one water-soluble block copolymer having at least one complexing anionic hydrophilic block and at least one stabilizing nonionic hydrophilic block; and
c) the partial or complete hydrolysis of the said precursor in order to obtain controlled growth of the mineral particles in the form of an aqueous dispersion.
According to a variant, the process of the invention furthermore includes, after step a):
a1) the partial hydrolysis of the product obtained in a).
The mineral precursors are, in general, mineral ions, mineral clusters, ultrafine particles whose particle size is less than 100 nm, or mixtures thereof.
More particularly, the mineral precursors are saltsxe2x80x94oxides or hydroxidesxe2x80x94 of a cation of a metallic element chosen from an element from column 3b (scandium column) to column 5 a (nitrogen column) of the Periodic Table of Elements, including the rare earths and the actinides. More particularly preferred within the context of the present invention are the multivalent metals or ions and more particularly the rare earths, in particular cerium, yttrium, precious metals and platinum-group metals, namely gold, silver, platinum, iridium, ruthenium, rhodium, osmium and palladium, transition metals, more particularly iron, cobalt and nickel, as well as copper, zinc and aluminium.
The optional partial hydrolysis of step a1) and the hydrolysis of step c) are preferably carried out using mineral bases such as alkali or alkaline-earth hydroxides, among which mention may be made of sodium hydroxide, ammonium hydroxide, potassium hydroxide and calcium hydroxide. If, when carrying out the process of the invention, it is necessary to use n moles of base to achieve the desired hydrolysis of the cation of the mineral precursor, it is recommended to use n1 moles of base during the optional step a1) followed by the use of n2 moles of base when carrying out step c), with the relationship n=n1+n2. The term xe2x80x9cdesired hydrolysisxe2x80x9d according to the present invention should be understood to mean the cumulative final hydrolysis obtained after implementation of optional step a1) followed by implementation of step c). This final hydrolysis may either be partial or complete. Complete hydrolysis is obtained when n is equal to the charge of the metal cation and results in an oxide or hydroxide present in the final mineral particle. If the final hydrolysis is partial, it is possible to obtain in the final mineral particle hydroxide salts such as, for example, Cu2(OH)3C1. In many cases, in order to obtain high-quality particles, it is recommended to use an approximately stoichiometric amount of base (n moles) in order to completely convert the anion or the metal contained in the mineral precursor into an oxide or hydroxide. According to a preferred variant, n, n1 and n2 are linked by the relationships:
0.2nxe2x89xa6n1xe2x89xa60.8n; and
0.2nxe2x89xa6n2xe2x89xa60.8n and
n1+n2=1.
Steps a), a1), b) and c) are generally carried out in an aqueous reaction solution whose temperature is room temperature (about 20xc2x0 C. ) and at atmospheric pressure, although a lower or higher pressure than this atmospheric pressure can be used. However, it is possible to raise the temperature of the reaction mixture to a temperature of between room temperature and the boiling point of this mixture, which is generally about 100xc2x0 C. Steps a), a1), b) and c) are carried out in an aqueous reaction solution whose pH is by preference set between 5 and 12, and preferably has a pH at least equal to the pKa of the anionic block of the copolymer.
The process according to the invention may include, immediately after step c), an additional step d) of maturation of the colloid dispersion at a temperature generally of between 50xc2x0 C. and a temperature of less than or equal to the boiling point of the said dispersion. Optionally, after step c) or d), the process according to the invention may include an additional step of concentrating the dispersion. This additional concentration step may especially be carried out by ultrafiltration, dialysis, drying/redispersion in water, precipitation/redispersion in water or centrifuging/redispersion in water.
More than approximately 80% of the colloid particles obtained at the end of step c) generally have a size of between 2 and 500 nm, preferably between 2 and 200 nm. One way of controlling the size of the colloid particles is to use a greater or lesser amount of moles n1 during step a) This is because the greater n1 and the closer it is to n, the greater the particle size.
The water-soluble block copolymer has at least one complexing anionic hydrophilic block and at least one stabilizing nonionic hydrophilic block. These copolymers have a number-average molecular mass {overscore (M)}n of preferably between 2000 and 20,000 and preferably between 3000 and 10,000 g/mol. Preferably, a copolymer having a nonionic hydrophilic block of greater mass than the complexing ionic block is used. The anionic blocks include, for example, polymethacrylic acid and its salts, polyacrylic acid and its salts, copolymers of methacrylic acid and its salts, copolymers of acrylic acid and its salts, heparin, polyphosphates and polyamino acids, such as polyaspartic acid, polyglutaminic acid, polymalic acid and polylactic acid. The preferred anionic blocks within the context of the present invention are blocks having carboxylic groups in the polymer chain. Examples of monomers allowing such blocks to be prepared are acrylic acid, aspartic acid, citraconic acid, p-hydroxycinnamic acid, trans-glutaconic acid, glutamic acid, itaconic acid, linoleic acid, methacrylic acid, maleic acid, oleic acid, maleic anhydride, mesaconic acid, 2-propene-1-sulphonic acid and vinylsulphonic acid.
The nonionic blocks, include, for example, polyether glycols, in other words polyethylene oxide, polypropylene oxide, ethylene oxide-propylene oxide copolymers, polysaccharides, polyacrylamides, polyacrylic esters, polymethacrylamide, polyvinyl alcohol, polyvinyl pyrrolidone, polyorthoesters, polyamino acids and polyglycerols. The preferred nonionic blocks within the context of the present invention are polyhydroxyethyacrylate and polyvinyl alcohol blocks. In order to prepare the block polymers, it is possible, for example, to use the anionic polymerization with sequential addition of 2 monomers as described, for example, by Scmolka, J. Am. Oil Chem. Soc. 1977, 54, 110; or else Wilczek-Veraet et al., Macromolecules 1996, 29, 4036. Another process that can be used consists in starting the polymerization of a block copolymer at each of the ends of another block polymer as described, for example, by Katayose and Kataoka, Proc. Intern. Symp. Control. Rel. Bioact. Materials, 1996, 23, 899.
Within the context of the present invention, it is recommended to use a living or controlled polymerization as defined by Quirk and Lee (Polymer International 27, 359 (1992).
This particular process makes it possible, in fact, to prepare polymers of narrow dispersity and the length and composition of the blocks of which are controlled by the stoichiometry and the degree of conversion. Within the context of this type of polymerization, more particularly recommended block co-polymers are those which may be obtained by any so-called living or controlled polymerization process such as, for example:
controlled radical polymerization using xanthates according to the teaching of Application WO 98/58974;
controlled radical polymerization using dithioesters according to the teaching of Application WO 97/01478;
polymerization using nitroxide precursors according to the teaching of Application WO 99/03894;
controlled radical polymerization using dithiocarbonates according to the teaching of Application WO 99/31144;
atom-transfer radical polymerization (ATRP) according to the teaching of Application WO 96/30421;
controlled radical polymerization using iniferters according to the teaching of Otu et al., Makromol. Chem. Rapid. Commun., 3, 127 (1982);
controlled radical polymerization using degenerative transfer of iodine according to the teaching of Tatemoto et al., Jap, 50, 127, 991 (1975), Daikin Kogyo Co Ltd, Japan and Matyjaszewski et al., Macromolecules, 28, 2093 (1995);
group transfer polymerization according to the teaching of O. W. Webster, xe2x80x9cGroup Transfer Polymerizationxe2x80x9d, pp. 580-588 in Encyclopaedia of Polymer Science and Engineering, Vol. 7 and H. F. Mark, N. M. Bikales, C. G. Overberger and G. Menges, Edit., Wiley Interscience, New York, 1987;
controlled radical polymerization using tetraphenylethane derivatives (D. Braun et al., Macromol. Symp., 111, 63 (1996)); and
controlled radical polymerization using organocobalt complexes (Wayland et al., J. Am. Chem. Soc. 116, 7973 (1994)).
Another means for controlling the size of the colloid particles is to vary the amount of complexing copolymer, which is the molar ratio of the complexing group of the complexing anionic hydrophilic block or blocks to the number of moles of the metal cation contained in the mineral precursor and which is converted into oxide or hydroxide. This ratio is generally between 0.05 and 10, more particularly between 0.1 and 1. In general, the higher this ratio, the smaller the size of the colloid particles.
Among the possible applications for the colloidal systems prepared by the process according to the invention, mention may be made of the mechanical polishing of hard objects, such as metal components, the production of pigments, of mixed ceramics for electronics, the reinforcement of polymeric matrices, fungicidal or biocidal dispersions and the scavenging of sulphur derivatives and, more generally, the trapping of unpleasant smells.
The following examples illustrate the invention without limiting the scope thereof.